The ability of various fluoride treatments to reduce dental disease is well documented, but the mechanisms involved are not well established. The mode of action of fluoride in promoting remineralization as well as reducing enamel solubility are being studied. A multi-disciplinary approach is being used so that both the solution mediated events and morphological as well as histological changes on or within the enamel may be investigated. The agents tested included sodium fluoride, ammonium fluoride, titanium fluoride and sodium hexafluorostannate. All agents were effective in increasing enamel fluoride levels when a minimal washing procedure was employed. Washing for 24 hours with either 1M KOH to remove soluble calcium fluoride or with "synthetic saliva" reduced the levels of fluoride to nearly control levels. The only treatment which showed consistent reductions in lesion formation was titanium tetrafluoride. Since the fluoride uptake from this treatment was less than that of APF we looked for morphological differences in the enamel. An acid resistant surface coating of unknown composition was deemed responsible for the reduction in lesion formation. In order to understand the mechanisms of action of fluoride necessary to prevent dental decay and not merely assume that "more will be better."